The present invention relates to an exhaust gas recirculation system for an internal combustion engine of an automotive vehicle. More particularly, the invention relates to an exhaust gas recirculation system in which the amount of recycled exhaust gas may be varied in response to operating conditions including engine speed, load conditions, warm-up conditions, vehicle speed, and temperature of intake air.
A known technique for reduction of emission of pollutants, particularly nitrogen oxides, in the exhaust gases of an internal combustion engine which are discharged to the atmosphere is to recycle a portion of the exhaust gases to a stage preceding the combustion stage, usually to the carburetor.
In a means according to one conventional approach, the air-fuel mixture is made leaner or richer than the theoretical air-fuel ratio of 15, at which nitrogen oxide emission is a maximum, and a comparatively small amount of exhaust gas is recirculated. However, there are definite limits to the amount of reduction of NO.sub.x emission that can be achieved by such a means.
To provide increased control of NO.sub.x emission in order to meet the requirements of government or other regulations without having an excessively adverse effect on average engine operating conditions, it has therefore been proposed to set the air-fuel ratio at the theoretical ratio and to greatly increase the amount of recirculated exhaust gas.
To achieve recirculation of the desired large amounts of exhaust gas it has been proposed to introduce the recirculated gas into a carburetor via separate ducts which are upstream and downstream of the throttle valve in the carburetor, i.e., upstream and downstream in terms of air flowing through the carburetor, and, in order to maintain the ratio of recirculated exhaust gas to the air-fuel intake more or less constant over the range of moderately low to moderately high load conditions for the engine to make the upstream supply of recirculated exhaust gas proportional to the air intake and the downstream supply proportional to the pressure downstream of the venturi section of the carburetor. In conventional means, control of the flow rates of recirculated exhaust gas is effected simply by orifices, and the large amount of exhaust gas recirculated by conventional means is very disadvantageous in certain operating conditions. In particular,
(1) Recirculation of a large amount of exhaust gas when the engine is rotating at high speed or is operating under a high load, or when vehicle speed is high, inevitably leads to reduced engine output and/or increased fuel consumption rates;
(2) Depending on temperature conditions within and around the carburetor, the contribution to a temperature increase made by exhaust gas recirculated to the upstream portion of the carburetor can be the cause of undesirable heating of the fuel float chamber, with consequent percolation and escape of fuel. Alternatively, depending on ambient temperature conditions and relative humidity of the intake air, recirculated exhaust gas, which has a high moisture content, may be the cause of icing in the carburetor.
To summarize, conventional means do not really consider relating the main problem to exhaust gas recirculation, namely how to recirculate the gas when definite advantages are achieved thereby, but stop recirculation of the gas the advantages are largely or completely outweighed by disadvantages relating to other aspects of engine operation.